The pic is adapted from the book by former BBC journalist Martin Sixsmith, titled “The Lost Child of Philomena Lee: A Mother, Her Son and a 50 Year Search.”

Dench will play Philomena Lee, with Coogan taking the role of Sixsmith. Coogan also co-wrote the screenplay with Jeff Pope.

The pic will shoot in late October, produced by Tracey Seaward, Gaby Tana and Coogan for Pathe, BBC Films and Coogan’s company Baby Cow. BFI is co-funding.

After a convent education that left her ignorant of the facts of life, Lee fell pregnant in 1952 and gave birth to a son at Sean Ross Abbey, a Catholic home for unmarried mothers in Tipperary.

Three years later, the nuns made her give up the boy, to be sold for adoption in America. Lee, who was forced to sign a document promising “never to seek to know” what happened to her son, later moved to England where she married and had a family. In 2004, she met Sixsmith, and together they embarked on a remarkable quest to track down her long-lost son.

They learned that the boy, whose birth name was Anthony, had been renamed Michael Hess, and grew up to become a lawyer and Republican politician in the 1980s. But he was also a closeted homosexual with AIDS. Lee discovered that, with his time running out, her son had travelled to Ireland to plead with the convent to reveal the identity of his mother so he could meet her before he died, but the nuns refused.

The book is described by its publishers as a “heart-breaking yet ultimately redemptive” story about a mother and son “whose lives were blighted by the forces of hypocrisy on both sides of the Atlantic.”

Coogan said: “The film is a comic tragedy or a tragic comedy. It’s about two very different people, at different stages of their lives, who help each other and show that there is laughter even in the darkest places.”

Shooting takes place in London, Northern Ireland and Washington, D.C.

Pathe is handling worldwide sales and distributing in U.K. and France.